1. Field of the Invention
The present invention relates to a bonding apparatus and more particularly to a bonding apparatus that includes a stage that supports a subject device and is grounded, a joining machine unit that joins a wire to the subject device, and a measurement unit that measures the connection state between the subject device and the wire.
2. Description of the Related Art
In a wire bonding apparatus used in assembling electronic components, a wire such as a slender gold wire is transported by a capillary to the bonding pad of a semiconductor chip supported on, for example, a bonding stage and bonded. The bonded wire is extended and is again bonded at the bonding lead of a circuit board, etc., thereby connecting the bonding pad of the semiconductor chip and the bonding lead of the circuit board. If bonding between the wire and the bonding pad or bonding lead does not go well, a connection defect known as so-called non-adhesion occurs.
In wire bonding, the bonding stage is usually metal, and the item to be bonded is supported this metal bonding stage. For example, if the item to be bonded is a semiconductor chip that includes transistors or diodes, the wire is connected to the electrode of the semiconductor chip with a resistance component, which corresponds to the characteristics of the semiconductor chip, between that electrode and the back of the semiconductor chip. Therefore, in order to detect non-adhesion, an appropriate direct-current current is inputted to the semiconductor chip from the wire, the flowing current value is measured, and the connection defect is judged electrically.
In the bonding apparatus disclosed in Japanese Patent Application Laid-Open (Kokai) No. H9-213752, an electrical path that extends from the side of a bonding arm that supports a capillary to the bonding stage is included in one side of an alternating-current bridge circuit that receives an alternating-current signal from an alternating-current signal generator, and non-adhesion between the wire and the bonded component is detected by detecting equilibrium or non-equilibrium of the alternating-current bridge circuit. In this method, non-adhesion even for a bonded component that has a capacitance component can be detected. The elements on three sides of this alternating-current bridge circuit are a fixed capacitance and a variable resistance and a fixed resistance; and the remaining one side is the impedance between the bonding arm side and the bonding stage. The initial equilibrium of the alternating-current bridge is found by adjusting the variable resistance to a state where there is no connection between the component to be bonded and the wire.
In some cases, the item to be wire bonded has no electrical conductance between the bonding stage. For example, in the case of a glass epoxy circuit board where a chip is mounted with die bonding, the board itself is an insulator. Therefore, the chip upon it is supported by the bonding stage with an insulator interposed. Also, a BGA (ball grid array) board too is a glass epoxy or ceramic material. In these cases, even if a capacitance component is not included between the back of the chip and the bonding pad, the electrical path from the bonding arm side to the bonding stage does not have direct current conduction.
In view of the above, it is preferable to measure using an alternating-current signal, as described in the above-described Japanese Patent Application Laid-Open (Kokai) No. H9-213752. However, measuring with an alternating-current signal in this system combines the capacitance of the mechanism that joins the wire to the subject device (when this is called a joining machine unit, the capacitance component of a joining machine unit) and the device-side capacitance, including the insulating substrate and the subject device. Therefore, when the capacitance component of a joining machine unit is large, the amount of change due to wire adhesion/non-adhesion is buried, and detection becomes difficult.
In Japanese Patent Application Laid-Open (Kokai) No. H9-213752, the capacitance component of the electrical path, which extends from the bonding arm of the bonding apparatus to the bonding stage, which is the capacitance component of the joining machine unit, could be compensated by adjusting the variable resistance. Of course, in this method also, it is possible in principle to find the equilibrium of the alternating-current bridge; but in practice, the capacitance component of the electrical path that extends from the bonding arm of the bonding apparatus to the bonding stage is not compensated, so that it is sometimes difficult to detect adhesion according to the minute changes in the capacitance component.